Light olefins, such as propylene and ethylene, are produced as co-products from a variety of feedstocks in a number of different processes in the chemicals, petrochemical, and petroleum refining industries. Various petrochemical streams contain olefins and other saturated hydrocarbons. Typically, these streams are from stream cracking units (ethylene production), catalytic cracking units (motor gasoline production), or the dehydrogenation of paraffins.
Currently, the separation of olefin and paraffin components is performed by cryogenic distillation, which is expensive and energy intensive due to the low relative volatilities of the components. Large capital expense and energy costs have created incentives for extensive research in this area of separations, and low energy-intensive membrane separations have been considered as an attractive alternative.
In principle, membrane-based technologies have advantages of both low capital cost and high-energy efficiency compared to conventional separation methods for olefin/paraffin separations such as propylene/propane and ethylene/ethane separations. Three main types of membranes have been reported for olefin/paraffin separations. They are facilitated transport membranes, polymer membranes, and inorganic membranes. Facilitated transport membranes, or ion exchange membranes, which use silver ions as a complexing agent, have very high olefin/paraffin separation selectivity and high olefin fluxes. However, poor chemical stability due to carrier poisoning currently limit practical applications of the facilitated transport membranes.
Separation of olefin from paraffin via conventional polymer membranes has not been commercially successful due to inadequate selectivities and permeabilities of the polymer membrane materials, as well as due to plasticization issues. Polymers that are more permeable are generally less selective than are less permeable polymers. A general trade-off has existed between permeability and selectivity (the so-called “polymer upper bound limit”) for all kinds of separations, including olefin/paraffin separations. In recent years, substantial research effort has been directed to overcoming the limits imposed by this upper bound. Various polymers and techniques have been used, but without much success. In addition, polymer membranes based on solution-diffusion separation mechanisms frequently suffer from plasticization of the polymer chains by the sorbed condensable penetrate molecules such as ethylene and propylene. Plasticization of the polymer, represented by the membrane structure swelling and a significant increase in the permeabilities of all components in the feed, occurs above the plasticization pressure when the feed gas mixture contains condensable gases, resulting in the decrease in selectivity. On the other hand, inorganic membranes, such as carbon molecular sieve and zeolite inorganic membranes, potentially offer adequate selectivities. However, they are brittle and currently too costly to be commercially useful for large scale applications.
Accordingly, it is desirable to provide processes for olefin/paraffin separation using membranes that have high selectivity and that are highly permeable. In addition, it is desirable to provide processes for olefin/paraffin separation using porous, hydrophobic poly(ether ether ketone) membranes. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.